Silver halide polyvinyl alcohol emulsions containing hardening agent



United States Patent US. Cl. 96--111 1 Claim ABSTRACT OF THE DISCLOSURE orthophosphoric acid, when added to a photographic emulsion comprising an aqueous solution of polyvinyl alcohol with silver halide dispersed therein, produces optimum hardening.

This invention relates to photographic emulsions and more particularly to emulsions in which synthetic binders containing polyvinyl alcohol are employed, and to hardeners for use therein.

Much research work has been carried out heretofore in a search for synthetic materials which would be free from the disadvantages of gelatin for use in photographic applications. A number of materials have been developed which have the thermally reversible gelling properties of gelatin dispersions, and which can be used as binders for photographic emulsions. Among these is polyvinyl alcohol, which can be used as such, or with other water dispersible materials as a photographic emulsion gelling agent and binder.

Emulsions containing polyvinyl alcohol require the addition of hardening agents so that after coating of the emulsion upon the support or substrate, the photosensitive material can be exposed and then further processed by the usual procedures using aqueous photographic processing solutions. While a number of such materials have been available heretofore, they have certain disadvantages, such as hardening the emulsion system too rapidly, thus interfering with coating techniques; insufficient permeability of the resulting emulsion layer after coating and drying, so that developing solutions could not penetrate, and the like. Another disadvantage of some of these hardeners is their failure to harden the emulsion coating sufficiently to provide satisfactory mechanical stability during processing.

It is an object of this invention to provide novel photographic emulsions which contain polyvinyl alcohol as a gelling agent and binder therefor, and a new hardening agent and photographic materials using such emulsions.

In accordance with the above and other objects of the invention, it has been found that by the use of orthophosphoric acid as a hardening agent for photographic emulsion systems containing polyvinyl alcohol, very useful photographic materials can be prepared. Aqueous emulsions thus prepared have a long pre-coating shelf life without irreversibly gelling or hardening, and they can be stored at ordinary temperatures fora long time before coating on a support. During coating, tackiness of the emulsion coatings is eliminated without appreciably increasing the viscosity of the photographic emulsion. After coating, the emulsion is sufiiciently hard to make it abrasion-resistant, while at the same time, the dry coating is permeable to aqueous solutions to such a degree that complete development, fixing and washing can occur. Additionally it is unexpectedly found that the photographic coating is further hardened during development when basic (alkaline) developer solutions are used.

3,453,113 Patented July 1, 1969 ICC Polyvinyl alcohol is prepared by the hydrolysis (alcoholysis) of polyvinyl acetate. This process is ordinarily not carried to completion and it is to be understood that the polyvinyl alcohol polymers used herein can contain a proportion of unhydrolyzed acetate groups.

Broadly speaking, the orthophosphoric acid is added to the photographic emulsion system containing polyvinyl alcohol during mixing of the emulsion and preferably prior to the addition of the soluble silver salt. Thus for example an aqueous mixture containing polyvinyl alcohol in amount sufiicient to provide the desired binder properties for the ultimate photographic emulsion is acidified to a pH of the order of about 3. Dilute phosphoric acid is then added slowly and with stirring to provide a homogeneous mixture. In the dark, a soluble silver salt, e.g., silver nitrate, is added in desired amount with stirring. Silver halide is formed in the usual manner. The emulsion may then if desired be kept at an elevated temperature for Ostwald ripening, following which it is coated, as on paper or a synthetic resin film substrate, or glass, in the usual way.

The amount of polyvinyl alcohol employed in any particular emulsion system may vary, depending upon whether or not it is the only gelling agent employed in the particular emulsion system used. Commonly, 5 to 10 parts of polyvinyl alcohol of medium molecular weight as used for gelling fluids per 100 parts of water are used when this is the only synthetic hydrophilic peptizing colloid employed. About 1 to 20 percent of orthophosphoric acid, based upon the weight of polyvinyl alcohol employed, is used as a hardener. Conveniently, the phosphoric acid is added as an aqueous solution of the commonly available percent orthophosphoric acid. For photographic purposes, reagent grade material is used to avoid undesirable impurities.

In non-photographic applications, solutions of polyvinyl alcohol can be used for sizing paper, coatings on paper and cloth, cement or glue, etc. These can be prepared in concentrations of upwards of about 1 percent by weight; and orthophosphoric acid when added in amounts of about 1 to 20 percent based on the dry weight of the polyvinyl alcohol strengthens and hardens the coatings made therewith. Accordingly, the invention comprehends within its scope aqueous solutions of upwards of one percent by weight of polyvinyl alcohol, containing from 1 to 20 percent by weight of orthophosphoric acid, based on the dry weight of the polyvinyl alcohol.

The molecular weight of the polyvinyl alcohol affects its water-solubility, as is known; lower molecular weight polymer may be soluble to as much as 20 to 30 percent by weight or more, while high molecular weight material is less soluble. The choice of molecular weight and concentration will depend on the end-use and one skilled in the art will have no difficulty in selecting the appropriate materials and amounts.

The invention will now be illustrated by the following examples, in which all parts are by weight unless otherwise specified.

EXAMPLE 1 A well stirred mixture of polyethylenimine, 5 parts (50 percent aqueous solution, molecular weight=30 40,000) and polyvinyl alcohol, 62.5 parts (8 percent aqueous solution, available under the trade name Elvanol 72-51) is acidified to pH 3 by the addition of hydrobromic acid. To the well stirred mixture is added slowly 2 parts of dilute phosphoric acid (made by adding 0.5 part of 85 percent orthophosphoric acid to 1.5 parts of water), with thorough mixing to insure homogeneity. The mixture is then placed in the dark, and a solution of 5 parts of silver nitrate in 5 parts of water is added with thorough mixing over a period of about seconds. The mixture is further stirred for about 3 minutes. To this is added 1 part of surfactant solution (12 percent aqueous solution of Poly- Tergent J-300, a nonionic, water soluble polyether) and the mixture is stirred for an additional minute. The resulting emulsion is knife coated upon paper base, in the dark, and upon plastic film support, the coatings are air dried and stored in a dark place. All of the steps in the preparation of the emulsion are performed at or near room temperature.

When exposed to light through a negative in the usual way, and developed with the ordinary paper developing solutions, excellent prints with good contrast are obtained, the surface of the paper being semi-glossy and resistant to abrasion even during processing in the aqueous solutions.

EXAMPLE 2 A polyvinyl alcohol solution (104 parts of 9.6 percent aqueous solution) is acidified by the addition of 10 parts of 48 percent aqueous hydrobromic acid. To this is added 4 parts of 21 percent aqueous phosphoric acid, with good stirring. Thereafter all operations are conducted in the dark. A solution of 10 parts of 5.9 N silver nitrate in water is added with good stirring over a period of about 10 seconds, and the mixture is then further stirred well for several minutes. To this is added 4 parts of surfactant (as used in Example 1) and the mixture is stirred for an additional 3 minute period.

Coatings of the resulting photographic emulsion are made upon baryta paper in the usual way. The coated paper is air dried and stored in light-proof containers.

On exposure to light, using the paper as a printing medi-,

um under a photographic negative, followed by standard development with metol-hydroquinone developer and fixing, using sodium thiosulfate fixing solutions, excellent prints are obtained. These show good mechanical resistance to abrasion, together with excellent permeability for the aqueous developing and fixing solutions.

EXAMPLE 3 To 100 parts of a 10 percent aqueous solution of polyvinyl alcohol prepared from high molecular weight polyvinyl acetate by hydrolysis, the degree of hydrolysis being about percent, are added, with stirring, 3 parts of 50 percent aqueous orthophosphoric acid (d=l.335, prepared by diluting percent orthophosphoric acid as commercially available). The solution, after mixing, can be stored for days without gelling or hardening, at room temperature. When coated upon paper, the coating is stronger and tougher than a similar coating prepared with the same polyvinyl alcohol solution which does not contain orthophosphoric acid.

What is claimed is:

1. A dispersion of silver halide and binder adapted for use to prepare a photographic emulsion, comprising an aqueous solution of polyvinyl alcohol containing a silver halide dispersed therein, together with from 1 to 20 percent of orthophosphoric acid, 'based on the dry weight of the said polyvinyl alcohol, dissolved therein, as a hardening agent for the said polyvinyl alcohol.

References Cited UNITED STATES PATENTS 2,376,371 9/1943 Lowe et al 96-114 3,113,026 12/1963 Sprung 96-107 J. TRAVIS BROWN, Primary Examiner.

J. R. HIGHTOWER, Assistant Examiner.

US. Cl. X.R. 96114 

